Multi-channel communication systems are often susceptible to interference between the various channels, also referred to as crosstalk or inter-channel crosstalk. For example, digital subscriber line (DSL) broadband access systems typically employ discrete multi-tone (DMT) modulation over twisted-pair copper wires. One of the major impairments in such systems is crosstalk between multiple subscriber lines within the same binder or across binders. Thus, signals transmitted over one subscriber line may be coupled into other subscriber lines, leading to interference that can degrade the throughput performance of the system. More generally, a given “victim” channel may experience crosstalk from multiple “disturber” channels, again leading to undesirable interference.
Different techniques have been developed to mitigate, suppress or otherwise control crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectrum management techniques to multi-channel signal coordination.
By way of example, certain of the above-noted techniques allow active cancellation of inter-channel crosstalk through the use of a precoder. In DSL systems, the use of a precoder is contemplated to achieve crosstalk cancellation for downstream communications between a central office (CO) or another type of access node (AN) and customer premises equipment (CPE) units or other types of network terminals (NTs). It is also possible to implement crosstalk control for upstream communications from the NTs to the AN, using so-called post-compensation techniques implemented by a postcoder.
One known approach to estimating crosstalk coefficients for downstream or upstream crosstalk cancellation in a DSL system involves transmitting distinct pilot signals over respective subscriber lines between an AN and respective NTs of the system. Error feedback from the NTs based on the transmitted pilot signals is then used to estimate crosstalk. Other known approaches involve perturbation of precoder coefficients and feedback of signal-to-noise ratio (SNR) or other interference information.
Crosstalk estimates are commonly utilized in situations where one or more inactive lines are being activated in a DSL system. The lines that are being activated are referred to as “activating lines” or “joining lines.” For example, it may become necessary to activate one or more inactive lines in a synchronization group that already includes multiple active lines, where synchronization in this context refers to alignment in time of the DMT symbols for the different lines. Such activating of an additional line may require that the crosstalk compensation be adjusted accordingly in order to optimize system performance. Exemplary techniques for controlling crosstalk associated with a joining line are disclosed in European Patent Application Publication No. EP 1936825A1, entitled “A Transient Crosstalk Controlling Device,” which is incorporated by reference herein. Crosstalk estimates are also used in other situations, e.g., as a means to track changes in crosstalk over time.
A given communication system may incorporate a variety of different types of CPE units that comply with different DSL standards. For example, some of the CPE units may be compliant with a particular vectoring standard, while other “legacy” CPE units do not comply with that particular vectoring standard. In such a communication system, it can be particularly difficult to determine estimates of the crosstalk from active vectoring-compliant lines into an activating legacy line. As a result, the activating legacy line may be unable to achieve full-rate activation, and performance of the system is adversely impacted. This situation arises in many practical communication systems, such as those in which CPE units are gradually being upgraded to support G.vector technology, which was recently standardized in ITU-T Recommendation G.993.5. It is expected that these systems will include a mixture of G.vector compliant and legacy CPE units for a significant period of time, particularly where remote software upgrades of CPE units are not desirable or feasible.